V4L/DVB (9557): gspca: Small changes for the sensor HV7131B in zc3xx.

- touch only one register for brightness change
- no quality control
- don't probe again at streamon time.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>